A sensor manufacturer's decision to change the sensors used for sensing the brightness of medical monitors has raised the issue of evaluation of brightness sensors. In the past, they have been evaluated by accuracy tests and field tests,but in the accuracy tests there have been problems of measuring equipment calibration and measurement error, and the criteria (e.g. pass/fail) by which operation is evaluated in field tests have been ambiguous. Another problem has been the lack of a method of determining the comprehensive relative merits of brightness sensors. In this study, measurement error was evaluated according to the JIS Z 9090 standard, which provides an absolute quantitative scale for determining measurement error. That made it possible to quantify the effects of error factors, thereby enabling the comprehensive relative merits of brightness sensors to be evaluated. It is expected that quantified error factor effects will be taken into consideration in future development work. This study also suggests the possibility of quantitative evaluation of other similar products.